This application is based on Application No. 2001-155299, filed in Japan on May 24, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a thermosensitive flow rate sensor including a heating element, the thermosensitive flow rate sensor measuring the flow velocity or flow rate of a fluid based on a heat transfer phenomenon where a heat is transferred to the fluid from the heating element or a portion heated by the heating element, the thermosensitive flow rate sensor being used to measure an intake air flow rate in an internal combustion engine, for example.
2. Description of the Related Art
FIG. 9 is a plan showing a detecting element used in a conventional thermosensitive flow rate sensor, and FIG. 10 is a cross section taken along line Xxe2x80x94X in FIG. 9 viewed from the direction of the arrows.
In FIGS. 9 and 10, a flat substrate 1 is constituted by a silicon substrate having a thickness of approximately 0.4 mm. An electrically-insulating support film 2 made of silicon nitride, etc., having a thickness of 1 xcexcm is formed on a surface of the substrate 1 by performing a method such as sputtering, chemical vapor deposition (CVD), etc. A heating element 4 constituted by a thermosensitive resistor film of platinum, etc., is formed on the support film 2. The heating element 4 is constructed so as to be formed with electric current paths by depositing a thermosensitive resistor film of platinum, etc., having a thickness of 0.2 xcexcm on the support film 2 by performing a method such as vapor deposition or sputtering, etc., and patterning the thermosensitive resistor film by using a method such as photolithography, or wet or dry etching, etc. A fluid temperature detector 5 similarly composed of a thermosensitive resistor film of platinum, etc., is formed on the support film 2 away from the heating element 4. The fluid temperature detector 5 is constructed so as to be formed with electric current paths by depositing a thermosensitive resistor film of platinum, etc., having a thickness of 0.2 xcexcm on the support film 2 by performing a method such as vapor deposition or sputtering, etc., and patterning the thermosensitive resistor film by using a method such as photoengraving, or wet or dry etching, etc. In addition, an electrically-insulating protective film 3 made of silicon nitride, etc., having a thickness of 1 xcexcm is formed by performing a method such as sputtering, CVD, etc., on the heating element 4 and the fluid temperature detector 5.
The heating element 4 is connected through first and second connection patterns 9a and 9b and first and fourth lead patterns 7a and 7d to first and fourth electrodes 8a and 8d for electrically connecting a detecting element to an external circuit. The fluid temperature detector 5 is connected through second and third lead patterns 7b and 7c to second and third electrodes 8b and 8c for electrically connecting the detecting element to an external circuit. The protective film 3 is removed from portions of the first to fourth electrodes 8a to 8d so as to be connected to an external circuit by a method such as wire bonding.
In addition, a flow rate detection diaphragm 12 is constructed by forming a cavity 13 under a region where the heating element 4 is formed. More specifically, a rear-surface protective film 10 is formed on a rear surface of the flat substrate 1 (a surface on the opposite side from the surface on which the support film 2 is formed), and then an etched hole 11 is formed by partially removing the rear-surface protective film 10 by a method such as photolithography at a position on the rear side of the region where the heating element 4 is formed. Thereafter, the flow rate detection diaphragm 12 is constructed by applying alkali etching, for example, to the flat substrate 1 exposed through the etched hole 11 to remove part of the flat substrate 1 and form the cavity 13.
The detecting element 14 constructed in this manner is disposed such that the flow rate detection diaphragm 12 is exposed to the flow of the fluid being measured. Moreover, in each of the figures, an arrow 6 indicates the direction of flow of the fluid being measured.
The detecting element 14 has a flat shape, as described above, and when the diaphragm 12 is disposed so as to be perpendicular to the direction of flow of the fluid being measured, fluid pressure acts on the diaphragm 12, giving rise to damage to the diaphragm 12 when the fluid being measured is flowing at high velocity, and dust in the fluid being measured may also accumulate on the diaphragm portion, changing the rate of heat transfer from the heating element 4 to the fluid being measured, thereby giving rise to drifts in the detected flow rate. In such cases, the flat detecting element 14 is disposed generally parallel to the direction of flow of the fluid being measured or so as to be inclined at a predetermined angle relative to the direction of flow of the fluid being measured.
When the flat detecting element 14 is disposed generally parallel to the direction of flow of the fluid being measured or so as to be inclined at a predetermined angle relative to the direction of flow of the fluid being measured, disturbances may arise in the flow of the fluid being measured in the vicinity of the cavity 13, or irregularities may arise in the flow of the fluid being measured in the vicinity of the heating element 4 due to irregularities in the shape of a leading edge portion of the detecting element 14 resulting from chipping, etc. These irregularities in the flow of the fluid being measured in the vicinity of the heating element 4 lead to decreased precision in flow rate detection.
Thus, in order to solve the problems described above in cases where the flat detecting element 14 is disposed generally parallel to the direction of flow of the fluid being measured or so as to be inclined at a predetermined angle relative to the direction of flow of the fluid being measured, it has been proposed in Japanese Patent Non-Examined Laid-Open No. 11-326000, for example, that the detecting element be disposed inside a recess portion formed on a flat support.
FIG. 11 is a partial perspective showing a support construction of the conventional detecting element described in Japanese Patent Non-Examined Laid-Open No. 11-326000.
In FIG. 11, a support 16 is formed into a flat shape, and is mounted to a base member 20. A recess portion 18 having a slightly larger external shape than the detecting element 14 is formed on a surface of the support 16. The detecting element 14 is disposed inside the recess portion 18 such that a surface of the detecting element 14 is positioned generally in a common plane with a surface of the support 16. The first to fourth electrodes 8a to 8d of the detecting element 14 are electrically connected by wires 19 to lead wires 17 disposed in the base member 20. A cover 21 is mounted to the base member 20, and the first to fourth electrodes 8a to 8d and the wires 19 are protected by the cover 21.
Hence, disturbances in the flow of the fluid being measured arising in the vicinity of the cavity 13 are suppressed, and the flow of the fluid being measured is smoothed by the arc shape of an upstream end portion of the support 16, reducing irregularities in the flow of the fluid being measured in the vicinity of the heating element 4 that are generated by irregularities in the shape of the leading edge portion of the detecting element 14.
Next, a method for detecting the flow rate of a fluid being measured using the detecting element 14 will be explained.
Because the fluid temperature detector 5 is separated from the diaphragm portion 12, heat generated by the heating element 4 is not transferred to the fluid temperature detector 5. Because the fluid temperature detector 5 is not positioned downstream from the heating element 4, the fluid temperature detector 5 is not exposed to the fluid being measured that has been warmed by heat transfer from the heating element 4. Thus, the temperature detected by the fluid temperature detector 5 is substantially equal to the temperature of the fluid being measured.
The heating element 4 is controlled by a detector circuit shown in FIG. 12 so as to be at a resistance value such that the average temperature of the heating element 4 is higher by a predetermined temperature (100 degrees Celsius, for example) than the temperature of the fluid being measured detected by the fluid temperature detector 5. The detector circuit is constituted by a bridge circuit including the fluid temperature detector 5 and the heating element 4. In FIG. 12, first to fifth resistors R1, R2, R3, R4, and R5 are fixed resistors, OP1 and OP2 are operational amplifiers, TR1 and TR2 are transistors, and BATT is an electric power supply. Except for the fluid temperature detector 5 and the heating element 4, the detector circuit is constituted by a detector circuit substrate (not shown).
The detector circuit controls the excitation current Ih flowing to the heating element 4 by functioning so as to generally equalize electric potentials at Point a and Point b in the figure. If the flow velocity of the fluid being measured is high, the temperature of the heating element 4 drops since the rate of heat transfer from the heating element 4 to the fluid being measured increases. Thus, the excitation current Ih required to keep the average temperature of the heating element 4 at the value higher by the predetermined temperature than the temperature of the fluid being measured increases. By detecting the excitation current as a voltage Vout at first and second ends of the third resistor R3, a flow velocity signal or a flow rate signal for the fluid being measured flowing through the inside of a passage having a predetermined passage cross-sectional area can be obtained.
Now, if Th is the temperature of the heating element 4, Ta is the temperature of the fluid being measured, Rh is the resistance value of the heating element 4, Ih is the excitation current flowing to the heating element 4, and Qm is the flow rate of the fluid being measured flowing through the passage in which the detecting element 14 is disposed, then Expression (1) is satisfied:
Ih2xc2x7Rh=(a+bxc2x7Qmn)xc2x7(Thxe2x88x92Ta)xe2x80x83xe2x80x83(1) 
where a, b, and n are constants determined by the form and layout of the detecting element.
Thus, by making (Thxe2x88x92Ta)/Rh uniform regardless of Ta, Ih becomes a function of Qm and the output corresponding to Ih becomes the detected flow rate output of the thermosensitive flow rate sensor.
The support construction of the conventional detecting element shown in FIG. 11 has the problems described below.
When the temperature of the fluid being measured changes, errors arise in the flow rate value detected by the thermosensitive flow rate sensor unless the temperature of the heating element 4 is adjusted in response to the temperature changes in the fluid being measured so that the temperature detected by the fluid temperature detector 5 swiftly tracks the actual temperature of the fluid being measured. For example, when the temperature of the fluid being measured rises, if a time lag occurs before the detection of the temperature by the fluid temperature detector 5, the temperature detected by the fluid temperature detector 5 will be lower than the actual temperature of the fluid being measured, making the temperature of the heating element 4 lower than the normal predetermined control temperature. In other words, the excitation current flowing to the heating element 4 falls below the normal electric current control value. Thus, the flow rate value detected on the basis of the excitation current flowing to the heating element 4 registers as a lower value than the actual flow rate of the fluid being measured.
However, because the temperature detected by the fluid temperature detector 5 cannot immediately track the actual temperature of the fluid being measured due to the heat capacity of the support 16, errors in the flow rate value detected by the thermosensitive flow rate sensor cannot be suppressed in this support construction.
Thus, in order to suppress the influence of the heat capacity of the support 16 on the temperature detection tracking of the fluid temperature detector 5, a support construction for a detecting element has been proposed in Japanese Patent Non-Examined Laid-Open No. 10-2773, for example, in which the detecting element is supported by a support such that a first end of the detecting element formed with a fluid temperature detector extends outward from the support. However, in the support construction of this detecting element, the flow of the fluid being measured is disturbed by a tip portion of the support and a tip portion of the detecting element because the support terminates in the vicinity of the heating element. The heating element is subjected to the influence of these disturbances in the flow of the fluid being measured, giving rise to problems such as the detected flow rate value becoming erratic.
In a thermosensitive flow rate sensor for measuring an intake air flow rate of an automotive internal combustion engine in particular, there are cases in which intake air temperature changes suddenly at the entrance or exit of a tunnel, making it necessary to be able to track these changes in intake air temperature swiftly.
The thermosensitive flow rate sensor for measuring the intake air flow rate of the automotive internal combustion engine is disposed in the piping which links a throttle valve and an air cleaner case. In general, since piping of this kind is not an ideal straight pipe upstream and downstream from the thermosensitive flow rate sensor, flow velocity distribution and the direction of flow are nonuniform. In applications of this kind for measuring the intake air flow rate of the automotive internal combustion engine, stable flow measurement becomes difficult with detecting element support constructions in which flow separation and vortexing of the fluid being measured occur easily in the vicinity of the heating element (or flow rate detector portion).
The present invention aims to solve the above problems and an object of the present invention is to provide a thermosensitive flow rate sensor enabling flow rate to be measured accurately without adversely affecting a detected flow rate signal even if the temperature of the fluid being measured changes, by directing the fluid being measured under a region where a fluid temperature detector is formed to swiftly acclimatize the region where the fluid temperature detector is formed to the temperature of the fluid being measured by forced convective heat transfer.
In order to achieve the above object, according to one aspect of the present invention, there is provided a thermosensitive flow rate sensor including:
a detecting element having:
a flat substrate;
a heating element and a fluid temperature detector each made of a thermosensitive resistor film and formed so as to be separated from each other on a surface of the flat substrate; and
a flow rate detection diaphragm formed by partially removing the flat substrate from a rear surface side under a region where the heating element is formed; and
a support having a recess portion for housing the detecting element formed on a surface thereof, the support being disposed such that the surface is inclined at a predetermined angle relative to a direction of flow of a fluid being measured,
wherein the detecting element is housed inside the recess portion and supported by the support such that a surface of the detecting element is positioned generally in a common plane with the surface of the support and such that a direction of alignment of the heating element and the fluid temperature detector is perpendicular to the direction of flow of the fluid being measured, and
a groove having a groove direction lying in the direction of flow of the fluid being measured is formed in the support so as to pass under a region where the fluid temperature detector of the detecting element is formed.
The fluid temperature detector may be formed at a first end side of the flat substrate, the detecting element being supported in a cantilever configuration by the support such that a first end portion of the detecting element including the region where the fluid temperature detector is formed extends into the groove.
The groove may extend to an upstream end portion of the support in the direction of flow of the fluid being measured.
The groove may extend to a downstream end portion of the support in the direction of flow of the fluid being measured.
A fluid temperature detection diaphragm may be formed by partially removing the flat substrate from a rear surface side under a region where the fluid temperature detector is formed.
A pipe-shaped detector passage may be provided for the fluid being measured to flow through, the support being disposed inside the detector passage so as to divide into two sections a passage cross section of the detector passage perpendicular to the direction of flow of the fluid being measured.